This invention relates in general to business-band transceivers, and in particular to such transceivers which include frequency-synthesizing means.
Business-band transceivers are typically employed for two-way communications involving public safety, industrial and business-related situations. Often it is a base station which must communicate with remote vehicles in the field in order to dispatch such vehicles to selected destinations. In order to monitor and control the use of various transmitting and receiving frequencies and to preserve some degree of privacy and isolation among various transceiver users, the Federal Communications Commission (FCC) assigns a single frequency to the business or service and in some cases a larger business may be assigned two frequencies.
When a unique crystal must be employed in order to achieve the FCC assigned frequency, the continued operation of the transceiver becomes completely dependent upon the presence of such a crystal. Consequently, in the event the crystal is damaged, lost or otherwise becomes unserviceable, the transceiver may be "lost" from service for extended periods of time while a new, unique crystal is ordered. In order to overcome this crystal replacement disadvantage, certain transceiver designs have gone to the use of a single crystal in combination with some type of frequency synthesizer so that the transceiver is not crystal-dependent to a unique crystal. That is, a variety of crystals are acceptable to create the assigned frequency and once a crystal is selected, the frequency synthesizer is changed accordingly. Frequency synthesizing also permits a repair shop to loan out a radio while a business's radio is being repaired and all that is required is to set the frequency synthesizer of the "loaner" so that the operating frequency of the transceiver corresponds to the assigned frequency for the particular business. One problem with frequency-synthesizing receivers is the inability to prevent inadvertent frequency change by the user. This result can easily occur when the controls for frequency setting are front panel controls and are readily accessible.
While there are a wide variety of transceiver designs, the following patents and publications represent a sampling of design concepts and available products.
______________________________________ Patent No. Patentee Issue Date ______________________________________ 3,487,311 Luhowy 12/30/69 3,983,484 Hodama 9/28/76 4,027,242 Yamanaka 5/31/77 4,061,973 Reimers et al. 12/06/77 4,070,626 Binder et al. 1/24/78 ______________________________________ Publication for Model No. Manufacturer ______________________________________ TR-7400A Kenwood FM-28 Clegg Communications Corp. TS-700SP Trio-Kenwood Communi- cations, Inc. ______________________________________
Luhowy discloses a transceiver with tunable phase-lock oscillator and tuning voltage generator. The local oscillator stage of this transceiver can be phase-locked to any one of a wide band of frequencies applied thereto from a digitally-tunable frequency synthesizer. The oscillator is course tuned by the same mechanism which tunes the synthesizer.
Hodama discloses a multichannel signal transmitting and receiving apparatus which comprises a single-reference local oscillator, a phase-locked loop including a phase comparator receiving as one input a signal resulting from a frequency division of the output of the local oscillator and a voltage-controlled oscillator. The single local oscillator is used for both signal reception and signal transmission and a frequency converter is provided outside of the loop of the phase-locked loop.
Yamanaka discloses a double superheterodyne receiver having a single local oscillator which supplies a signal of a predetermined frequency to a second frequency converter or mixer where a second intermediate frequency signal is produced for final detection. The local oscillator signal is also supplied by way of a frequency divider to a phase-locked loop frequency synthesizer as a reference frequency signal.
Reimers et al. discloses a synthesizer which employs phase-lock loop techniques to generate either the carrier signal for an associated transmitter or the injection signal for an associated receiver. A programmable divider controls the specific channels to which the synthesizer tunes the transmitter or receiver whereas operation of the transmit or receive mode is dependent upon which one of two oscillators is coupled to the synthesizer circuit.
Binder et al. discloses a radio-telephone device which includes a channel selector containing at least one frequency divider having a plurality of control inputs each associated with a respective frequency dividing ratio and each arranged to receive an electrical signal for influencing its frequency dividing ratio.
The Kenwood model TR-7400A is a two-meter transceiver which provides front panel controls for selection of one of its 800 channels. The Clegg model FM-28 is also a two-meter transceiver which provides front-panel frequency select controls. The Trio-Kenwood model TS-700SP includes a plug-in crystal as well as the previously mentioned front panel controls for frequency selection. While each of the various transceivers may provide certain advantages, they do not overcome the disadvantage of placing the frequency-select controls in an accessible location where the frequency can be easily, though inadvertently, changed.